58 



COE 



The blood is dark red, and the blood vessels may be traced in the 

 living worm the whole length of the body. 



Habitat. — The species was found only at Kadiak, where it was 

 dredged in about three fathoms. It slightly resembles some varieties 

 of T. vermiculus Quatrefages, but the longitudinal bands of dark 

 pigment between the two ocelli of the same side are lacking, and the 

 median white line is sharply demarkated, so that the two species are 

 undoubtedly specifically distinct. 



20. TETRASTEMMA ABERRANS sp. nov. 



This is a minute Nemertean, the specimens found not exceeding 12 

 mm. in length in greatest extension. Body moderately slender, slightly 

 flattened. A pair of slight vertical slits on sides of head. 

 Color. — Pale yellow throughout, both above and below. 

 Ocelli. — Of moderate size, or rather small, arranged in four groups 

 which form a rectangle, as do the single eyes of typical species of the 

 genus. Each of the four groups is composed of three to five ocelli of 

 variable size. The two anterior groups lie well toward the tip of the 

 snout, while the two posterior groups lie above or slightly in front of 

 the brain (fig. 15). The appearance of the eyes 

 is such as to give at once an impression that the 

 multiple nature of each group has arisen from the 

 fragmentation of single ocelli. This I consider to 

 be the case. At least one other species of Tetra- 

 ste77ima ( T. cruciatjim Burger ) is known in 

 which the four ocelli are double, and I have 

 often noticed other species of the genus — notably 

 the fresh water T. rjibrinn (Leidy) — which 

 showed almost conclusively that one or more of 

 the six or seven eyes present had been derived 

 from a splitting of the primary ocelli. In one 

 instance the fragmentation had been carried so far 

 that no fewer than 30 to 30 pigment spots were 

 present. 



Proboscis sheath afid proboscis as in other 

 species of the genus. Mouth and proboscis open 

 together ; proboscis sheath reaches the posterior end of the body. Pro- 

 boscis provided with a rather slender central stylet and basis ; the two 

 accessory stylet pouches each with two or three st)-lets of typical form. 

 The proboscis possesses twelve nerves, and in this respect again shows 

 a departure from the typical Tetrastemma, where there are usually 

 but ten proboscidial nerves. 



Fig. 15. Tetrasiem- 

 ma aberrans. Dorsal 

 view of anterior por- 

 tion of body, showing 

 outline of brain and ar- 

 rangement of the four 

 groups of ocelli. X 25. 



